1. Field of the Invention
The present invention relates to a multi-spindle machine tool having separately and vertically movable first and second spindle units mounted on a main body thereof which is movable toward and away from a workpiece, and to a tool changer suitable for use in the multi-spindle machine tool.
2. Description of the Related Art
In an NC (numerically controlled) machine tool, a workpiece is normally machined by a single spindle. It has therefore been pointed out that a problem in such an NC machine tool is that when a workpiece needs to be subjected to a plurality of machining processes, the efficiency of machining is reduced. Many improvements in the NC machine tool have been made to solve such a problem. A machine tool having two pairs of spindle heads mounted on a single column is now used as has been disclosed in Japanese Laid-Open Patent Publication No. 59-69242, for example.
In the prior art, however, the two pairs of spindle heads are supported on the same column so as to be vertically movable by an actuator. Further, distances between respective tools fixed to the respective spindle heads are fixed. Therefore, the respective tools cannot be set so as to be adaptable to workpieces of different kinds, thereby causing a problem that such a machine tool cannot be used widely. Further, when there exist two portions in a workpiece to be machined which are close to each other, because of the difficulty in simultaneously machining the two portions, the machining efficiency is reduced.
Therefore, the present applicant has proposed a two-spindle machine tool which has first and second spindles supported by a movable table which is slidably supported on guide rails mounted on a main body of the machine tool. The proposed machine tool is also provided with a feed screw mechanism capable of adjusting the distance between the first and second spindles (see Japanese Laid-Open Utility Model Publication No. 62-138515). In the aforementioned machine tool, however, the movable table to which the respective spindles are fixed, extends in the direction of an axis of each spindle. It is therefore impossible to increase the supporting area of the movable table to be supported by the main body via the guide rails without interfering the workpiece. Thus, the rigidity of the movable table remains low and the use of a large spindle in particular will create a difficulty.
There has been used a tool changer of a type wherein a number of different tools are accommodated in a tool magazine and a desired tool is taken out from the tool magazine so that a tool which has been mounted on a spindle head of a machine tool is exchanged with a desired tool to meet various machining requirements. As has been disclosed in Japanese Laid-Open Utility Model Publication No. 63-110336, for example, there has been proposed an apparatus wherein a tool magazine for accommodating a plurality of tools therein is provided in the vicinity of a bed on which a main body having a plurality of main spindles is movably placed and a tool is automatically exchanged with a desired tool between the main spindles and the tool magazine.
In the above prior art, however, a guide member projecting toward the main body from the tool magazine is provided. Further, a chuck unit is provided so as to be movable along the guide member between the tool magazine and the main spindles. Therefore, the range of movement of the main body in its transverse direction is considerably restricted, thereby causing a problem that the range of machining of a workpiece by each spindle is limited.
In view of the foregoing problem, the present applicant has proposed a tool changer disclosed in Japanese Laid-Open Patent Publication No. 1-205945. However, this type of tool changer is used to simply exchange a single tool with another. Therefore, the tool changer cannot be used to simultaneously exchange a plurality of tools with others.